I don't know - I reached it through direct experience (a number of years programming professionally in a language with a good type system) rather than anything I read.
https://spin.atomicobject.com/2014/12/09/typed-language-tdd-... makes the case that many tests can be replaced more effectively by types (though the use of Java makes the point less clear than it should be, and means that some properties are so difficult to express through types that the author prefers to use tests). That one can prove correctness with types is well-known, and it stands to reason that there is no need for tests for formally verified code. These two things together suggest that there's never a point on the defect rate/cost curve that's easier to reach with tests than with types, but they don't rule it out entirely.
https://spin.atomicobject.com/2014/12/09/typed-language-tdd-... makes the case that many tests can be replaced more effectively by types (though the use of Java makes the point less clear than it should be, and means that some properties are so difficult to express through types that the author prefers to use tests). That one can prove correctness with types is well-known, and it stands to reason that there is no need for tests for formally verified code. These two things together suggest that there's never a point on the defect rate/cost curve that's easier to reach with tests than with types, but they don't rule it out entirely.